How to change the font of a PDF file, when it’s an original one?

When I was a kid, and the PC was in infancy, probably around year 1987, I programmed along with a fellow hacker a program called SFONT that changed the system font of a MS-DOS prompt. He made a very nice font, which imitated the science fiction (sci-fi) console. I liked to use it very much when programming, since it increased my productivity and imagination. He is a genius. I understood later, while learning neuroscience at Aalto University, that the characters can affect your mood and irritation level. The legibility is changed, with each font.